I love this.
It has been a long time since I have cruised a M ship, I'm not sure what the Mercury would be classified as but that was our first ever cruise, then we followed up with the Millennium the next year. We are booked on the Constellation this coming January and really looking forward to it. Don't let the age of a ship throw you or the size of the ship. My DW and I like the smaller ships. I study the floor layout before our cruise to get and idea, but there isn't anything like actually walking around and seeing for yourself. It eliminates a lot of frustration on trying to get from one area to another, if you know you need change a floor to get there.
I will add what I have always said and my adult kids live by, learn to go with the flow.
EVERY trip will have things pop up that are not great, how you deal with it makes your trip. Remember, everyone with you is on vacation and want to have a good time.